Seneca Crane was executed after Katniss's and Peeta's suicide stunt at the end of the Games, and the act was intended to remind the Gamemakers of his fate and suggest that they could suffer the same.

Why did Katniss show Seneca Crane on the dummy?

On the dummy, she writes Seneca Crane. Seneca Crane is the name of the last head Gamemaker. He was killed after allowing Peeta and Katniss to both win the previous Hunger Games. Katniss does this to remind them that they, even though head hanchos for the capitol, are not safe from President Snow's wrath.

Why was the gamemaker killed in the Hunger Games?

Seneca Crane used to be the Head Gamemaker, but when he let Katniss Everdeen and Peeta Mellark both survive the 74th Hunger Games, he was killed on orders of President Snow for allowing both Katniss and Peeta to live.